I have seen cases where giving the IP phones there own internet fixes issues. Best tool to do some basic troubleshooting s a continuous ping from end point to the IP card. If you get replies over 100ms, then you have call quality issues.
How are you prioritizing traffic? You have the IP card behind the router with the rest of the network?
Difserv is one way to proritize, but as TadiranTechTexas said, it may not play nice with cable internet. I have often seen no or one way audio when turned on.
